monitoring of the corrosion on a steel sheet-pile marine

Journal of Civil Engineering and Architecture 12 (2018) 39-50 doi: 10.17265/1934-7359/2018.01.004

Monitoring of the Corrosion on a Steel Sheet-Pile Marine

Breakwater by Systematic Thickness Measurements

Luis Osmel Millan Solorzano

Department, Millan Engineering, San José, 30303, Costa Rica

Abstract: In Quepos, Pacific of Costa Rica, it was finished on 2010 the first phase of a marina, including two mix breakwaters, with rubble mound (rocks and concrete units), and 25 circular steel sheet piles cofferdam cells, filled with sand and gravel. The maintenance plan, considers tracking sheet pile corrosion, comparing “actual” against expected rates, checking structural limits, and programming countermeasures if accelerated corrosion is identified. Specific control sections, along the breakwaters, both inside and outside the basin, were established. In each section, thicknesses were measured every meter from the top of the steel cell to seabed using an ultrasonic equipment, and an underwater transducer. Both land crew, and divers for submerged portions, were used. The measurements campaigns are for several years from 2011 to 2016. Sectors of the breakwater with varied corrosion attack levels could be differentiated. Also, corrosion rates and lifespans were estimated, both general for the structures, and specific for each section and level. In turn, this allowed to identify maintenance priorities, defining sites where measures of corrosion protection should initiate, as well, to have confidence in the structural capacity and safety of the breakwaters.

Key words: Monitoring of structures, maritime works, sheet piling, corrosion, ultrasonic thickness measurements.

1. Introduction

1.1 Objective1

The maintenance monitoring follows the corrosion

experienced by cellular steel cofferdam breakwaters

of a marina, tracking the corrosion of the sheet-piles,

and comparing the “actual” against the expected

corrosion rates, checking that the structural limits for

thickness are not exceeded, and programming

countermeasures in case that are identified potential

areas of accelerated corrosion.

1.2 Descriptions

The marina breakwaters surrounding the basin (or

the inside), are mixed, with combinations of rubble

mound slope breakwaters (rocks and precast concrete

units), and steel plain sheetpile cells in circular

arrangements, known as cellular cofferdams (Figs. 1

Corresponding author: Luis Osmel Millan Solorzano, civil

engineer, maritime and ports consultant; research fields: corrosion and cathodic systems, geotechnical and structural instrumentation, static and dynamic pile testing. E-mail: [email protected].

and 2).

Some of the cells were covered with rubble mound

(concrete dolosses and rock), or have 1-3 m high

concrete parapets on top, to complete the required

height for swell, or both. The lengths of these

breakwaters are:

737 m breakwater at north (named North

Breakwater) with a 353 m rubble mound section, and

16 circular cells of 18.6 m in diameter;

219 m at south-east (called South Breakwater)

with a 60 m rubble mound section, and 9 circular cells,

1 of 18.6 m in diameter and 8 of 12.2 m.

The sheet piles of the cells are Arcelor AS-500 type,

500 mm width. The thicknesses vary as per the

diameter of the cells, being 11.0 mm on the 12.2 m

cells, and 12.7 mm on the 18.6 m cells.

The steel of these sheet piles is ASTM A690, which

is “marine” steel, with a yield strength of 345 MPa. It

is recognized that on the splash zone (wave and tidal

exchange), this steel type has a corrosion resistance of

2-3 times more than normal steel, but not in the

submerged part, where normal and marine steel have

similar corrosion rates [1].

D DAVID PUBLISHING

Monitoring of the Corrosion on a Steel Sheet-Pile Marine Breakwater by Systematic Thickness Measurements

40

Fig. 1 Aerial view from 2010 of the Marina in Quepos.

Fig. 2 Arrangements and numbering for the cells and arcs.

The sheet piles were not protected by a barrier, i.e.,

with no paint or coatings prior to their installation, nor

with concrete or other material once constructed. Also,

no anodes were placed for cathodic protection of the

submerged parts. As a result, of these design decisions,

corrosion is expected to occur without restrictions.

Therefore, the designer considered for the tidal and

splash zone, as well for the submerged part, an

over-thickness that could corrode during the lifespan

of the structure, without affecting the capacity of the

cells. This leads to the need to follow up the corrosion

of the sheetpiles, verifying if the thicknesses are

within secure limits.

2. Method Statement

2.1 Measurement Sections definitions

Several control sections were chosen around the

breakwater cells, in a number of one section per cell

or arch inside and two sections per cell or arch outside,

with the purpose of being controlled annually.

This is done in this way to have a general

distribution of the measurements around the

breakwaters, and to consider different exposition

conditions, for the cells, inside and outside the basin.

Also, other few measurements were done behind

rock revetments, provisionally withdrawing rubble


41

mound, and in diapraghm sheet piles, which are the

cell elements within the fill, by previously excavating

the gravel and sand. This was made to understand the

behavior of these sections compared to exposed cells.

A nomenclature was adopted to define every

measurement location, including the cell number, the

specific sheet-pile (starting from the joint), and

measuring the height from the top of the cell down. At

each elevation, 4 points were measured as shown in

Fig. 3, so that minimum and average values could be

considered for statistical purposes.

Defining the measurements in such way allows

re-staking each section and points in a simple way, so

that they could be repeated during annual campaigns.

When the measurements were above the water,

access to the measuring points was done with stairs

and platforms. On the other hand, measurements

below the water were made with the help of divers.

2.2 Cleaning of Measurement Points

An air needle powered by a compressor was used at

each measurement point to clean the sheet-pile surface

from marine life and corrosion, at each measurement

point. The cleaning was done in a circle with no more

than 10 cm in diameter.

Fig. 4 shows this activity for underwater

measurement points. This cleaning was executed also

above water, in a similar way (i.e., with the same

equipment).

2.3 Measurements

The thickness measurements themselves were made

with a UT (ultrasonic equipment), having a nominal

frequency of 5 MHz, and a straight ½ in. diameter

underwater transducer, with a 15 m in cable, so that

the measuring device was at the upper part of the cell

all time.

Fig. 5 shows a measurement with the underwater

equipment. For comparison reasons and easiness, the

same underwater transducer was used for the above

water measurements.

2.4 Measurement Campaigns

The last measurement campaign, included in this

Fig. 3 Scheme of points measurements at each elevation.

Fig. 4 Underwater cleaning of measurement points.


42

Fig. 5 Underwater thicknesses measurement with ultrasonic sensor.

paper, finished on April 2016, when the sheet piles

had from 7-8 years of being installed. being

constructed. Other measurement campaigns were

executed before, as indicated:

June to August of 2011;

September of 2011;

January of 2012, measurements of arches and

internal and external cells;

March 2012, measurements on a section outside

the breakwaters, behind the rubble mound;

June 2012 thicknesses measurements in the

diaphragm wall, or inside the fill of the cells;

February 2013;

January 2015.

3. Results

3.1 Minimum Structural Thicknesses

To calculate the minimum admissible thicknesses,

design loads are used to estimate the sheet pile hoop

tensions, using the procedure from the Corp of

Engineers and Pile Buck Manuals [2, 3]:

⋅⋅=

..,

..

8.0min ,

, FS

ft

FS

RF ywsk

Rdt

MEdmEdt rpF ⋅= ,,

RdtEdt FF ,, ≤

where,

Pt, Rd is the admissible tension;

rm is the cell radius;

tw is the sheet-pile thickness;

fy is the sheet-pile material yield stress;

pm,Ed is the maximum tension, which can be

calculated with several formulas;

Pt, Ed the maximum cell tension force/length;

Rk, s interlock tension force;

S.F. is a safety factor.

For simplicity, the pressure in the splash zone was

considered half of the pressure in the immersed zone,

the last calculated as per the analytical formulation.

Critical elevations could vary from cell to cell and

could be either above or underwater. Corrosion levels

are higher at tidal zone, but tension forces diminish.

Opposed to this, below water, the highest tension of

the sheet-pile occurs around or just above the seabed

level, but corrosion is lower.

It must be recognized that, joint interlock tension

loss due to corrosion is difficult to measure, so an

alternative approach was considered.

It was assumed a relationship between the measured

thickness and the maximum tension that theoretically

can support the connection. This is based on

ARCELOR design manual as per Ref. [4].

Then, the thickness of the sheet pile corresponding

to the tension calculated for the joint is extrapolated.


43

The safety factor used in the formulas was 1.5.

In Table 1, are summarized minimum thicknesses in

the inner and outer exposed cells of main and

connecting arches, as well for the connecting yees (or

joints).

In addition to the cell filling, other interactions, such

as the presence of concrete parapets, or the effect of

external waves, may be neglected, since they do not

affect toward the outside of the cell and the resultant

hoop tension of the sheet-piles.

It could be concluded from the previous table that

the thicknesses for having failure, compared with the

theoretical thicknesses of the piles, are in fact low,

about 3-4 times higher.

Thicknesses are greater for the joints compared to

other sheet-piles. For the 18.6 m cells, sheet-piles

above water require higher thicknesses compared to

those below water, and the opposite is true for the 12.2

m cells.

3.2 Thicknesses Measurements

The measurements from 2016, for the inside of the

north breakwater, could be seen graphically in Fig. 6.

In this graph, one section is considered per cell, so in

cases with two sections measured per cell,

conservatively the one with lower thicknesses was

included in the figure. Levels are in meters from the

LLWS (lowest water level spring) so that effects of

tides are recognized. Similar graphs could be

considered for the other exposition conditions i.e.

south breakwater, and outside the basin.

From these graphs, specific cells with lower

thicknesses, i.e., higher corrosion could be identified,

for example, above +1 m LLWS inside cells 1-5 and

above +1 LLWS outside cells 1-8, with more corrosion

on cell 6 around 0 LLWS.

Also, statistically, the distribution of quantity of

measurements for given ranges is also considered, as

shown in Fig. 7. This graph shows the thickness

measurements distribution for the north breakwater

outside the basin, but graphs are similar for other

exposition conditions.

Inside the north breakwater, 49% of measurements

above LLWS and 60% below LLWS are between

12.0-12.5 mm, with a minimum measured thickness of

9.8 mm.

Fig. 6 Thicknesses (mm) at different elevations (from LLWL) and sections (one per cell) north breakwater inside, 2016.

Table 1 Minimum thickness (mm) to comply with the cell design hoop tension and a 1.5 safety factor [5].

Location 12.2 m cells 18.6 m cells

Outside

Above water Sheetpile 1.8 1.4

Joint 2.5 1.9

Below water Sheetpile 1.4 2.4

Joint 2.2 3.4

Inside

Above water Sheetpile 1.3 1.6

Joint 2.0 1.9

Below water Sheetpile 1.2 2.5

Joint 1.9 3.5


44

Fig. 7 Thicknesses measurements distribution for the north breakwater outside of the basin, 2016.

Fig. 8 Set of all thicknesses measurements inside the north breakwater vs. construction time 2012 to 2016.

Meanwhile, outside the north breakwater, 35% of

measurements above LLWS are from 11.5-12 mm, and

49% below LLWS are from 12-12.5 mm, with a

minimum measured thickness of 10.6 mm.

From previous, and as expected, corrosion attack is

higher above LLWS, and lower below it.

3.3 Thicknesses versus Time

The measurements of the four campaigns carried out,

were plotted against the years between measurement

and sheetpiles construction. This comparison considers

the generalized behavior of sheet piles over time.

Fig. 8 shows the case corresponding to the inside

part of the north breakwater. Similar cases were

addressed for the other conditions, i.e., north and south

breakwater, and inside or outside the basin.

Because the construction of the breakwater cells was

executed over a period of several months, the graph


45

ends up having a distribution of points that allows to

validate the observations and the calculations made in

this way.

As expected, it is concluded that the general

behavior of the sheet-piles after the construction of the

breakwaters is loss of thickness (or corrosion), this no

matter the exposition condition, North or South

breakwater, inside or outside the basin.

In general, in the north and south breakwaters, the

lower thicknesses are in the sheet piles above +0 m of

the lowest spring tide (LWWS). The trend seems to be

that higher up on the sheet-pile corrosion is greater.

In the north breakwater, the losses outside and

inside the basin are in the same magnitude order. But

on the south breakwater, this cannot be concluded

because there is only one section in the inner part

because rubble mound is laying in almost all the

internal cells.

Inside the basin, there is more corrosion in the

curved part of the north breakwater, compared to the

rest of the sheet piling. Outside this same breakwater,

the corrosion is greater on the most exposed cells to

waves. Outside the south breakwater, and depending

on height, corrosion is concentrated on central cells.

3.4 Thicknesses Differences

Besides the general comparison, differences

between data of the same measurement points from

2012 and 2016, were calculated.

For the same point, the average differences are from

0.46 to 1.10 mm and maximum from 1.22 to 2.61 mm.

Those differences are summarized in Fig. 9.

As shown, few measurements were higher in 2016

compared with the ones from 2012. That may be due

to differences in equipment or cleaning, but also

because, by procedure, measurements are directly on

site, which never happens in the same exact spot.

This is, for each measurement, an area of the sheet

pile surface about 10 cm diameter, is cleaned, and the

transducer is placed within this area. As the surface is

irregular, some differences between measurements

from different years are expected.

3.5 Corrosion Rates

Linear best fit lines were determined, with the

measurements from 2012 to 2016 as part of the same

set of data. For considering lower limits for the best

fits, two other parallel lines to the fit line were

included with a separation between them of 0.5 mm.

Fig. 9 Thicknesses differences of measurements, 2016-2012.


46

Fig. 10 shows the data and adjustment lines for the

inside section of the north breakwater. The same was

done for other combinations north-south breakwater,

inside-outside sections and over-under water.

These comparisons are intended to consider the

general behavior of the breakwater. Low correlations

in settings are expected, as they include different

levels and locations along breakwaters, where

individual corrosion rates are not the same.

The slope of the adjustment lines can be considered

as an average corrosion rate of the structure section. In

the case, for the external north breakwater above the

water, it is 0.23 mm/year, which is high, but expected

for tropics and no barrier or cathodic protection.

On the other hand, the estimates of the corrosion

rates for each of the measured points, are based

whether on the average of the measurements in each of

the elevations or the minimum in the same section and

elevation.

For all the cases, the corrosion rates averaged from

0.11 to 0.26 mm/year, which are high. It should be

clarified that these rates have been calculated with a

four-year term (2012-2016), and it is expected that the

estimates will improve over the years, and more data.

3.5 Estimated Lifespan

Considering that the best fit lines, their parallel lines,

and intersection with the minimum thicknesses, is

possible to establish general lifespans (or useful lifes)

for the steel breakwaters, considering each of the

analyzed exposition conditions (combinations of

north-south breakwaters, inside-outside the marina, or

above-below sea level).

Lifespans can also be calculated based with the

differences between the average measurements of 2012

and 2016 for each point. For each cell, the difference

between the current measurement of the point, and the

minimum safe thickness according the structural

calculation, is a remnant of corrosion (available

thickness that could corrode without failure of the

structure).

The time in years required for the estimated

measurement thickness to corrode the remaining

material up to the minimum thickness, in the

calculated corrosion rate, is related to the lifespan for

the sheet pile at that specific point.

The previous is summarized in Figs. 11 and 12,

respectively for the north breakwater inside and the

Fig. 10 Adjustment lines for the north breakwater outside and above waterline, 2012 to 2016.


47

Fig. 11 Lifespans (years) different elevations (from LLWL) and sections (one per cell) north breakwater inside, 2016.

Fig. 12 Lifespans for individual measurements at the north breakwater outside, 2016.

same breakwater outside. From these graphs it could

be inferred that most of the points would have

lifespans over 30 years.

However, there are critical cases with lifespans

between 10-20 years. Among the conditions

under-over water and inside-outside the marina, the

conditions over water and outside the marina have

lower lifespans.

For example, in the north breakwater, 86-89% of

inside cells have lifespans of more than 30 years, with

minimum individual lifespans form 15-20 years.

Meanwhile, outside 58-69% of cells have more than 30

years with minimum individual lifespans 10-15 years.

6. Future Activities

6.1 Measurements Follow-up

Sheet piles should be monitored in accordance with

the guidelines explained herein. Comparisons with

new annually campaign measurements, must be done.

In this regard, measurements, that will have more time

from the baseline (year 2012), will help determining a

more exact corrosion rate for the sheetpiles.

As sheet piles are losing thickness on the future,

more attention should be paid to subsequent thickness

measurement campaigns to detect problems.

6.2 Sheetpile Protections

Quantifying the conditions of the cell piles, by

sections and points, including the general and specific

lifespans, makes it possible to establish the

maintenance priorities for the steel cofferdams.

The protection procedures vary per area and position

of the sheet piles. For example, if they are in the splash

zone, tidal interchange zone, or submerged zone, or if


48

they are sheet piles of the main, arch or diaphragm.

Based on the current results, is being recommended

only the application of a barrier protection in the areas

above water with higher overall corrosion. This could

vary if there are important changes in corrosion rates

or behavior in future campaigns.

6.3 Tidal and Splash Coatings

Prior to the application of coatings and related

products, a surface preparation is required. All sheet

piles must be clean of oil, grease, any foreign material

or loose material, as well as any other contamination

that could affect adhesion of the coating to be applied.

For preparation of substrates, standards based on

SSPC (The Society for Protective Coatings) or the

NACE (National Association of Corrosion Engineers)

should be followed. Performance testing of products

are per the ASTM (American Society for Testing and

Materials).

It is recommended that the surface is prepared to

standard SSPC-SP-10 (SA 2.5; NACE 2) “surface near

white metal” using abrasive cleaning or flushing.

Mechanical means such as abrasive discs, scalers or

other devices may be used, if they are able to produce

deep cleaning of the surface, as required.

Wire brush cleaning should be avoided as it usually

spread the contamination, and not remove it. It must be

tried to remove even the contamination of surfaces

with oil, applying if it is the case powder soap or liquid,

since this affects the adhesion of the coatings.

Regarding the coatings themselves, suitable and

resistant products must be used for the application,

whether they can be applied and cured on and under

water. Cofferdams which allow the insulation and

extraction of water from a section of the wall could be

used for “dry” applications of the coatings.

In all cases, the recommendation is to follow the

manufacturers’ technical sheets regarding safety

standards, surface preparation, product mixtures,

application and curing conditions of such coatings.

Once installed, the coatings should be inspected

visually, and with instruments for detecting possible

defects. Mainly thicknesses measures of dry film, to

compare with the minimum and maximum

recommended by the manufacturer, as well as of

continuity of the coatings on the applied surface.

6.4 Tidal and Splash Concreter Covers

For concrete covers, to warranty the complete

adhesion of the concrete to the surface, it required, in

addition to the cleaning (which does not need to be

made to white metal), the placement of shear

connectors, which are welded steel elements to sheet

piles, for anchorage, and to support an internal

reinforcing steel mesh. Thicknesses of covers are

variable and starts from 10 cm and up.

The dimensions, thicknesses of the coatings as well

as the shear and reinforcing mesh connectors must be

designed. Surface cleaning can be performed prior to

formwork and final coating casting, as required.

It is recommended to use concrete with a minimum

resistance of 350 kg/cm2, due to the direct exposure

condition due to the waves and sea currents. The usual

controls must be carried in the manufacture, placement

and curing of these concrete covers.

Primarily, the segregations and washes of the

concrete should be avoided, including if necessary, the

use of impermeable forms, in which the water is

withdrawn from them. Attention should be paid to the

construction joints, which should be conveniently

waterproofed by means of water-stops, tapes, paints

and others.

6.5 Underwater Cathodic Protection

The area and location of the surface to be protected,

water resistivity, and time of protection, drive the size

and quantity of sacrificial anodes required per design.

The anodes are placed on the surface, which, when it is

in contact with the sea water, produces an electric

current between the anode and the surface. Anodes are

the elements that corrode and wear, instead of the steel

surface that they protect. Accordingly, they are usually


49

referred to as sacrificial anodes.

The installation procedure, discussed in this section,

is by means of hot underwater welding, although other

methodologies could be used, if they warranty the

electrical continuity between the surface of the sheet

piling and the installed anode.

The surfaces of the existing sheet pile where anodes

will be placed, must be cleaned either by manual

means (rackets and wire brushes), and/or pneumatic

and hydraulic procedures (compressor and grinders or

escalators).

The area must be clean of marine life, and corrosion

products. The cleaning must be such that there is a free

minimum area around anodes legs, so that they could

be welded.

The anodes must be lowered to the water by

mechanical and safe methods, so that neither personnel

on the ground nor divers take risks or lift unnecessary

weights. Each anode to be installed, must be placed at

the location where it will be welded.

If necessary, temporary structures may be used,

which should not damage the sheet pile. Care should

be taken not to leave a gap between the anode holding

leg and the surface of the sheet pile.

With the anode secured in the position, it is welded.

Safety standards should be applied, mainly the use of a

switch to discontinue the current when the welding

electrode is not in use, and direct reverse current in the

polarity of the welding machine.

On the surface, it is advisable to make a record of

several aspects of welding as location, polarity,

amperage, voltage, electrode characteristics, and so on.

Also, welds must also be inspected after defects.

Finally, it should be checked if the anodes give the

minimum required protection to the sheet-piles, by

measuring the electrical potentials generated by the

system. The measurement of these potentials must be

done using a multimeter and reference electrode. The

negative phase is connected to the reference electrode

in the water, and the positive part to an attachment

connecting the sheet pile. A maximum negative

potential must be met at different heights in the water

to be protected.

7. Conclusions

For the maintenance follow-up of a steel sheet piles

cells of a mixed breakwater, at a marina in Quepos,

Puntarenas, Costa Rica, several thickness yearly

measurements with ultrasonic equipment, were

executed in campaigns from 2011 to 2016.

The purpose of the control is to verify the condition

of the cells, at north and south breakwaters, both inside

and outside the marina, and below and above waterline.

From the comparison, of 2016 measurements with

those of the previous campaigns, it is possible to

determine that the phenomenon of corrosion is being

presented in all the steel sheet piles.

However, this corrosion is not homogeneous and

there are different behaviors for the outside and inside

cells of the marina, above and below the water, as well

along the breakwaters.

At first, external sheet pilings tend to have more

corrosion than internal ones, probably because they are

being more attacked by waves. Also, more degree of

corrosion is given on the sheet piles in the splash zone,

and within this zone, the corrosion is greater and has

more dispersion in the upper part of the cell.

General corrosion rates are estimated based on the

regression of the absolute measurements of the years

2012-2016, to estimate the overall trend.

Also, specific rates for each elevation were obtained

from the averages and minimum values of the

measurements points, taking as the initial thickness of

the sheet pile, the measurement of 2012.

Thus, the difference is obtained with the

measurements of 2016, which represents the thickness

losses by corrosion between those years.

The remaining lifespan is calculated, from the date

of the last campaign (April 2016). The corrosion limit

is the calculated minimum thicknesses, by tension of

the sheet piles or joints.

The calculation of useful life from general trend data,


50

is compatible with the requirement for the structure of

at least 30 years. On the other hand, considering each

section and specific level, there are cases with smaller

lifespans, including isolated cases with 10-20 years,

mainly on sectors identified as critic.

By quantifying the sheet-pile thicknesses on

different sections and levels, it is possible to identify

critical sections. For these sections specific and

directed solutions, could be accounted for, to protect

those specific sections of sheet-piles, rather than apply

general solutions.

In maintenance, this in turn, allows to make a more

efficient, economic and safe use of the resources,

principally applying them when they are required.

In general, the protection of sheet-piles could be

done, above water in the intertidal and splash zones by

barriers (coatings or concrete cover), and below water

using cathodic protection.

At current time, the protection from corrosion, of the

steel surfaces, by a barrier is what is recommend, since

structurally thicknesses are above for what is required,

as minimum as calculated by design.

References

[1] Coburn, S. K. 2003. Corrosion Factors to be Considered

in the Use of Steel Piling in Marine Structures. Pittsburgh:

Pile Buck, Inc.

[2] U.S. Army Corps of Engineers. 1989. EM 1110-2-2503

Design of Sheet Pile Cellular Structures Cofferdams and

Retaining Structures. Washington, DC, 1989.

[3] Pile Buck, Inc. 1990. Cellular Cofferdams. Jupiter,

Florida: Pile Buck, Inc.

[4] Arcelor RPS (Rails, Piles & Special Sections). Arcelor

Group. 2005. Piling Handbook. 8th ed.

[5] Bardi, J., and Abam, B. 2012. Marina Pez Vela Cellular

Cofferdam Evaluation. Washington: Berger Abam.

monitoring of the corrosion on a steel sheet-pile marine

Documents